Accurate fast start up power on reset

I have a requirement were I need to get a processor up and running very qui ckly. 10ms in fact. Most power on reset circuits have a large variation on reset time of 5 to 15ms. I am looking for something that is accurate to sa y 20% and simple and cheap. Threshold of around 2.7V. I do have a 32kHz clo ck that appears when power above 2.8V which could possible be used, any ide as would be appreciated.

Reply to
steve
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10ms from what? A power supply being plugged in? A physical power switch being turned on? A wake-on-LAN message? And how quickly are your power supplies going to go from 0 to ready?

I ask because all that stuff bakes in additional ambiguity. ESPECIALLY anything mechanical, which might chatter for milliseconds just on its own. Speaking of mechanical things, how reliable is the start time of your 32 kHz clock?

If this requirement really is managable, I'm going to guess that your best answer will be something with a shunt VREF, an open-drain comparator, and an RC. The LM393 predates cave paintings, but is dirt cheap and a fine choice for this; half to make a ramp and half to square it up.

Reply to
Rob Gaddi

Den mandag den 7. december 2015 kl. 11.43.29 UTC+1 skrev steve:

uickly. 10ms in fact. Most power on reset circuits have a large variation o n reset time of 5 to 15ms. I am looking for something that is accurate to say 20% and simple and cheap. Threshold of around 2.7V. I do have a 32kHz c lock that appears when power above 2.8V which could possible be used, any i deas would be appreciated.

ldo with a powergood output ?

-Lasse

Reply to
lasselangwadtchristensen

1) Use a 555 for turn-on timing. Simplest and least expensive. Power on signal can be microseconds after threshold. 2) Have that oscillator run a ripple counter, say divide by at least 16 (the oscillator is doing half of the work,the divide by 16 ensures the oscillator is stable. Power on signal would be 0.5mSec after oscillator on at divide by 16, 1.0mSec delay at divide by 32,etc.
Reply to
Robert Baer

quickly. 10ms in fact. Most power on reset circuits have a large variation on reset time of 5 to 15ms. I am looking for something that is accurate t o say 20% and simple and cheap. Threshold of around 2.7V. I do have a 32kHz clock that appears when power above 2.8V which could possible be used, any ideas would be appreciated.

10ms from the 3V rail reaching 2.7V, the 32KHz clock is from an RTC chip th at is permanently running. Its 32KHz output appears when the power rail hit s 2.8V when its switches from battery to 3V power. The psu reaches 2.7V in less than 5ms. An external device triggers the PSU to start up and 30ms lat er it sends its data to be captured by us. Hoipe that makes sense
Reply to
steve

Just curious, how much searching did you do? There are tons of reset supervisors out there in small packages and low prices.

How about the MAX6394? It has a 2.9 volt version with a 1 to 2 ms timeout.

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Rick
Reply to
rickman

ry quickly. 10ms in fact. Most power on reset circuits have a large variati on on reset time of 5 to 15ms. I am looking for something that is accurate to say 20% and simple and cheap. Threshold of around 2.7V. I do have a 32k Hz clock that appears when power above 2.8V which could possible be used, a ny ideas would be appreciated.

Y

p that is permanently running. Its 32KHz output appears when the power rail hits 2.8V when its switches from battery to 3V power. The psu reaches 2.7V in less than 5ms. An external device triggers the PSU to start up and 30ms later it sends its data to be captured by us. Hoipe that makes sense

Rick I need 10 ms +/- 20%, this chip does 14ms min 28ms max which is the us ual problem of a 3:1 min to max ratio

Reply to
steve

What prevents you from being ready before 10ms - 20% ?

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-TV
Reply to
Tauno Voipio

If you cannot find a single chip to do that do it in two chips, say one of those no-timer "power good" detectors and a 555 or a 74hc123 or something. Will take some passives but I guess this is the price to pay for the accuracy you are after. Typically no one cares about another 10 or 100 ms longer time reset being held active, e.g. I had to look in the datasheet of the part I have been using for years to check if it would work for you (it would not). BTW those readily available one chip parts are fine as long as you use them as specified. Many years ago I used a Maxim part and strayed a little - made some assumptions about unspecified behaviour - and *all* my assumptions proved wrong... I ended up removing the part on the second board revision and doing things myself somehow (don't remember, was 20+ years ago), adapting to the single chip like I did on the first revision was more pain than that.

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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Reply to
Dimiter_Popoff

A PIC10F320 will run from 2V to 5.5V and has an ADC, a voltage reference, and an accurate internal oscillator which wakes up in a few microseconds.

The rest is firmware.

Cheers

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Syd
Reply to
Syd Rumpo

very quickly. 10ms in fact. Most power on reset circuits have a large varia tion on reset time of 5 to 15ms. I am looking for something that is accurat e to say 20% and simple and cheap. Threshold of around 2.7V. I do have a 3

2kHz clock that appears when power above 2.8V which could possible be used, any ideas would be appreciated.

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hip that is permanently running. Its 32KHz output appears when the power ra il hits 2.8V when its switches from battery to 3V power. The psu reaches 2.

7V in less than 5ms. An external device triggers the PSU to start up and 30 ms later it sends its data to be captured by us. Hoipe that makes sense

e usual problem of a 3:1 min to max ratio

It is possible 5ms minimum 10ms max. 5ms is the amount of time the processo rs fast clock takes to start. From experimentation 5ms was ok 3ms was not, so 10ms seems a could place

Reply to
steve

I didn't understand. I thought you just needed it to come up *within*

10 ms. You need an exact time delay? Someone suggested a 555 timer.

But that is 10 ms from reaching 2.7 volts? That will require a comparator circuit with a good reference. The 555 has the comparator if you add a good reference. But that will need its own supply since it has to be up first I think.

You said the 32 kHz output is enabled at 2.8 volts, so if you need a time from 2.7 volts that won't help.

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Rick
Reply to
rickman

g very quickly. 10ms in fact. Most power on reset circuits have a large var iation on reset time of 5 to 15ms. I am looking for something that is accur ate to say 20% and simple and cheap. Threshold of around 2.7V. I do have a 32kHz clock that appears when power above 2.8V which could possible be use d, any ideas would be appreciated.

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chip that is permanently running. Its 32KHz output appears when the power rail hits 2.8V when its switches from battery to 3V power. The psu reaches

2.7V in less than 5ms. An external device triggers the PSU to start up and 30ms later it sends its data to be captured by us. Hoipe that makes sense
t

the usual problem of a 3:1 min to max ratio

sors fast clock takes to start. From experimentation 5ms was ok 3ms was not , so 10ms seems a could place

can't the processor wake up on the slow clock and switch to the fast when it is ready?

-Lasse

Reply to
lasselangwadtchristensen

ing very quickly. 10ms in fact. Most power on reset circuits have a large v ariation on reset time of 5 to 15ms. I am looking for something that is acc urate to say 20% and simple and cheap. Threshold of around 2.7V. I do have a 32kHz clock that appears when power above 2.8V which could possible be u sed, any ideas would be appreciated.

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TC chip that is permanently running. Its 32KHz output appears when the powe r rail hits 2.8V when its switches from battery to 3V power. The psu reache s 2.7V in less than 5ms. An external device triggers the PSU to start up an d 30ms later it sends its data to be captured by us. Hoipe that makes sense

set

s the usual problem of a 3:1 min to max ratio

essors fast clock takes to start. From experimentation 5ms was ok 3ms was n ot, so 10ms seems a could place

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The processor needs at least 5ms of slow clock, which is 32768 whilst held in reset. It could be done using the 32768 clock, this is always running bu t its output is enabled when supply reaches 2.8V. This could drive a divide by 256 ripple counter, if such a thing existed in a very small package oth er wise an accurate time reset circuit, maybe using a voltage detector and a 555 type circuit.

Reply to
steve

It's not clear how much time happens between the power supply crossing

2.7 volts and crossing 2.8 volts, but a simple counter chip will allow the 32 kHz to be used for accurate timing from the 2.8 volt crossing... assuming the clock chip has an *accurate* 2.8 volt detector. Which clock chip?

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The three counters in the above data sheet give you binary divides from the 32 kHz. The reset would need to be asserted until power is up enough to work which may require a reset detector chip with no delay or the one I specified with minimum delay might do. You get binary multiples of slightly less than 1 ms. The CD4020 would do well and there is an HCT equivalent.

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So this might be 2 chips, but the reset chip is pretty minimal. The

74HC(T) is available in a QFN so it's durn small and probably cheap as well.
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Rick
Reply to
rickman

MAXIM???? Are you nuts? They have been known for ages to announce things they (for practical purposes) do not make, drop "product" like a hot potato with no notice, etc. Any other manufacturer, including none is better and far more reliable.

Reply to
Robert Baer

Well this is not entirely fair. Much of it true may be, but Maxim have also had some good products for some years. And they have always been helpful with samples to me when they could not deliver small quantities.

For example, they (still?) are the source of various fast comparators, nobody else could match them when I needed them over the years (not quite recently but I used some again 2-3 years ago). Then they used to have good references - nowadays ADI beat them (and ADI just do not drop products, which is truly great) but I have used theirs for years (in small quantities indeed). More to the point - reset circuits - I'd probably agree with you though. Many years ago I used a MAX695 - which worked as specified but I misused it somewhat (in a sane way), and all my misuse went wrong (unlike other times when I have misused parts). Was no drama but was not nice either.

Dimiter

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Reply to
Dimiter_Popoff

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